1. Field of the Invention
The present invention relates generally to oscillators and more particularly to crystal oscillators having both negative and positive feedback loops.
2. Discussion of Related Art
Many oscillator designs have been suggested in which a high Q, series resonant circuit, such as a piezoelectric crystal is used to produce frequency stability. Such oscillators are used as reference frequency generators in many applications. A primary consideration in these applications is that the oscillator output be stable against changes due to voltage, temperature and secular changes.
U.S. Pat. No. 3,996,530 to Feistel et al shows a Butler Oscillator having an amplitude limiting amplifier following a filter network interposed between the voltage amplification stage and the impedance matching stage in order to ensure that the crystal is driven by a sinusodial waveform, free of distortion, for maximum frequency stability.
U.S. Pat. No. 3,836,873 to Healey, III et al discloses a cascode amplifier configuration comprising first and second transistors in combination with inductance and capacitance elements to provide an oscillator configuration with phase shift in the vicinity of the oscillator frequency dominantly controlled by the quartz crystal unit motional impedance parameters.
U.S. Pat. No. 3,878,481 to Healey, III discloses a VHF oscillator circuit having first and second transistors, a first resonant circuit coupled to each of the collectors of the first and second transistors, and a transformer for coupling energy from the first resonant circuit in a manner that signals substantially equal in amplitude but opposite in phase are applied respectively to the bases of the first and second transistors. In addition, an anti-resonant circuit comprising a crystal unit is coupled to the emitter of the first transistor.
U.S. Pat. No. 3,569,865 to Healey III discloses a high stability voltage controlled oscillator having a series coupled emitter follower transistor amplifier with regenerative feedback to a tuned circuit coupled to the base electrode of the amplifier. The tuned circuit has a narrow pass band crystal filter in series with a variable capacitor and in parallel with a low Q inductor with a controlled voltage adapted to be coupled to the variable capacity device to vary the capacitive reactance of the tuned circuit to produce oscillation over a limited frequency range with high stability.
U.S. Pat. No. 3,571,754 to Healey III discloses a variable frequency harmonic oscillator including a voltage tunable crystal controlled resonator incorporating a quartz crystal unit with precisely anti-resonated static capacitance operating substantially at the series resonant frequency of the quartz as opposed to the anti-resonant frequency thereof and a voltage variable reactance network coupled thereto having a linear reactance vs. voltage characteristic.
U.S. Pat. No. 3,824,494 to Gerum discloses a crystal oscillator for use in clocks and watches.
U.S. Pat. No. 3,911,378 to Buchanan discloses a voltage controlled oscillator employing three series TTL inverting gates.